Subway Cave in Lassen National Forest was created by a lava flow that covered the Hat Creek Valley some 30,000 years ago. The 1/3 mile long lave tube is the largest accessible lava tube created by this seismic event and is definitely worth checking out if you plan on visiting nearby Lassen Volcanic National Park.
The Lassen National Forest lies at the heart of one of the most fascinating areas of California, called the Crossroads. Here the granite of the Sierra Nevada, the lava of the Cascades and the Modoc Plateau, and the sagebrush of the Great Basin meet and blend.
First off, lets get one thing straight, Subway Cave is located in Lassen National Forest which is not within the boundaries of Lassen Volcanic National Park. Subway Cave is the most visited location in the Hat Creek area, and may be reached by a side road just north of the highway 89-44 junction, ending at a parking area with interpretive notices, rest rooms and picnic tables.
The second largest deposits of ancient lava in California (after Lava Beds National Monument) are found just north of Lassen Volcanic National Park; a black, jagged, mostly treeless expanse stretching for over 20 miles.
A very short walk leads to the cave entrance, where steps enable an easy descent to the caves floor.
The passage extends both ways; on the left the cave has a rather low ceiling and is soon blocked by a fence, but the main section is to the right.
The walkable part is only about a 1/3 of a mile, but this is more than long enough to be completely dark for some distance in the middle, so make sure you bring some flashlights.
The roof of the cave is smooth apart from small lavacicles, where lava dripped slowly down before cooling, while the floor is generally flat, formed of solidified lava ripples.
Into the darkness.